1. Field of the Invention
The present invention relates to a vacuum booster used for the servo operation of a brake master cylinder for an automobile. The present invention particularly relates to an improvement in a vacuum booster comprising: a booster shell; a booster piston accommodated in the booster shell to divide an interior of the booster shell into a front vacuum chamber connected to a vacuum source and a rear operating chamber; a valve cylinder slidably supported at a rear wall of the booster shell to be connected to the booster piston; an input rod movable forward and backward, an input return spring that biases the input rod in a retreat direction, and a control valve that changes over communication of the operating chamber with one of the vacuum chamber and atmosphere in accordance with forward and backward movement of the input, which are rod are placed in the valve cylinder; the control valve being constituted by an atmosphere introduction valve seat that is formed on a valve piston which moves linking to the forward and backward movement of the input rod, a vacuum introduction valve seat that is connected to the valve cylinder to move linking to forward and backward movement of the valve cylinder, and a valve element that is biased by a spring in a direction in which it is seated on the atmosphere introduction valve seat and the vacuum introduction valve seat, cuts off communication of the operating chamber with the atmosphere and vacuum chamber when the valve element sits on both the valve seats, allows the operating chamber to communicate with the atmosphere when the atmosphere introduction valve seat opens following the forward movement of the input rod, and allows the operating chamber to communicate with the vacuum chamber when the vacuum introduction valve seat opens following the retreat of the input rod; and a reaction force mechanism that is interposed between the valve cylinder with the input rod and the output rod that is slidably supported at the booster shell, and that transmits a combined force of a manipulation input to the input rod and a propulsive force of the booster piston by atmospheric pressure difference between the operating chamber and the vacuum chamber to an output rod, wherein a valve piston is fitted onto an input piston of the reaction force mechanism which is connected to the input rod, so that the valve piston is capable of sliding forward from a predetermined retreat position with respect to the input piston, wherein a set spring that biases the valve piston to the retreat position is provided between the input piston and the valve piston, and wherein locking means is provided between the valve piston and the valve cylinder to hold the atmosphere introduction valve seat in an open state by locking the valve piston, which moves forward with the input piston, at the valve cylinder at the time of emergency braking when the input piston moves forward by a predetermined number of strokes or more ahead of the valve cylinder.
2. Description of the Related Art
Japanese Patent Application Laid-open Nos. 2001-63551 and 2001-341632 disclose conventional vacuum booster boosters, wherein at the time of emergency braking, the atmosphere introduction valves are opened large to introduce a large quantity of atmosphere into the operating chambers quickly so as to allow the output rods to exhibit a high output force.
In the vacuum booster disclosed in Japanese Patent Application Laid-open No. 2001-63551, a valve piston is fitted onto an input piston, which connects an input rod and a reaction force mechanism, so that the valve piston is slidable between a retreat position and an advance position along an axial direction with respect to the input piston; and a control valve is constituted by an atmosphere introduction valve seat that is formed at the rear end of the valve piston, a vacuum introduction valve seat that is formed in a valve cylinder to enclose the atmosphere introduction valve seat, a cylindrical valve element that has a common valve portion disposed to be opposite to and capable of being seated on the atmosphere introduction valve seat and the vacuum introduction valve seat at a front end and is mounted to the inner peripheral surface of the valve cylinder, and a valve spring that biases the valve portion in the direction in which the valve portion is seated on the atmosphere introduction valve seat and the vacuum introduction valve seat. Also, the valve cylinder is provided with a first port that communicates with the vacuum chamber and opens to an outer periphery of the vacuum introduction valve seat and a second port that communicates with the operating chamber and opens between the atmosphere introduction valve seat and the vacuum introduction valve seat, the inside of the valve element is allowed to communicate with the atmosphere, the valve piston is biased to the retreat position with a smaller set load of the return spring than the valve spring, and delay means for delaying the retreat of the valve piston by the biasing force of the return spring from the advance position is provided between the valve piston and the input piston. According to this constitution, at the time of emergency braking when the input rod is rapidly advanced, the valve piston receives the delay effect of the delay means and advances with the input piston while substantially keeping the advance position, and therefore the atmosphere introduction valve seat of the valve piston is separated greatly from the valve portion of the valve element simultaneously with the advance of the input rod and is opened to the maximum, whereby a large quantity of atmosphere is introduced into the operating chamber, and a large advancing propulsive force caused by the large atmospheric pressure difference between the vacuum chamber and the operating chamber is given to the booster piston immediately to raise a large output force of the output rod quickly, thus responding to the emergency braking. In addition, expensive solenoid device and emergency braking sensor are not required, and therefore the effect of contributing to reduction in cost is provided.
However, in this vacuum booster, the valve piston closes the atmosphere introduction valve seat in the state in which the valve piston is located at a predetermined advance position with respect to the input piston at a halt time, and therefore when the input piston advances, the atmosphere introduction valve seat cannot be opened unless the valve piston moves to the retreat position with respect to the input piston. Accordingly, the advancing strokes of the input piston until the valve piston is moved to the retreat position from the advance position become invalid strokes, and the atmosphere introduction into the operating chamber is delayed more or less. The vacuum booster disclosed in Japanese Patent Application Laid-open No. 2001-341632 has the same disadvantages.